Aluminium object with anodic oxide surface

ABSTRACT

There is provided an aluminium shaped object with an oxidised surface, characterised by an elastic aluminium oxide surface layer with a thickness of between 1 and 20 Mu m, preferably of 2 to 10 Mu m, with a spongy structure of granular structural elements having an average diameter of between 1 and 10 Mu m, preferably of 3 to 8 Mu m. There is also provided a process for the production of an aluminium shaped object with an oxidised surface by anodic oxidation in an electrolyte, wherein the surface of an aluminium shaped object is contacted in a weakly acidic to weakly alkaline bath by sparkovers at a high voltage and very high current density.

United States Patent [1 1 Lauten'schlager et al.

[4 1 Jan. 28, 1975 ALUMINIUM OBJECT WITH ANODIC OXIDE SURFACE [75] Inventors: Werner Lautenschlager, Stuttgart; Siegfried Pahlke, Bobingen/Rcms; Gunther Tolg, Schwabisch-Gmund, all of Germany [73] Assignee: Max-Planck-Gesellschaft zur F orderung der Wissenschaften e. V., Gottingen, Germany [22] Filed: Jan. 22, 1973 [21] App]. No.: 325,765

l'2/l966 McNeil! et al. 204/58 9/l970 Suzuki 204/58 Primary Examiner-R. L. Andrews Attorney, Agent, or Firm-Burgess, Dinklage & Sprung [57] ABSTRACT There is provided an aluminium shaped object with an oxidised surface, characterised by an elastic aluminium oxide surface layer with a thickness of between I and 20 ,um, preferably of 2 to 10 pm, with a spongy structure of granular structural elements having an average diameter of between 1 and 10 um, preferably of 3 to 8 am.

There is also provided a process for the production of an aluminium shaped object with an oxidised surface by anodic oxidation in an electrolyte, wherein the surface of an aluminium shaped object is contacted in a weakly acidic to weakly alkaline bath by sparkovers at a high voltage and very high current density.

9 Claims, 3 Drawing Figures ALUMINIUM OBJECT WITH ANODIC OXIDE SURFACE BACKGROUND OF THE INVENTION.

Aluminium shaped objects with oxide coatings are known. They are usually produced by anodic oxidation in acidic electrolytes, for example, in chromic acid. sulphuric acid, oxalic acid or the like by the so-called eloxal process. A disadvantage of the layers produced by this process is their brittleness which. in the case of subsequent working up, leads to a cracking and flaking off of the oxide layer. The structure of the oxide layers is smooth so that they are unsuitable for many purposes, for example, as carrier materials for chromatographic separations.

Furthermore, processes are also known for the chemical oxidation of aluminium in which the shaped objects are dipped into solutions of weakly oxidising substances. In this case, very thin protective coatings are admittedly obtained but, for many purposes, the clasticity of these layers is insufficient. In addition, the structure is not absorption-active so that layers of this type also cannot be used, for example, for chromatography.

It'is an object of the present invention to provide an aluminium shaped object with a uniform surface of aluminium oxide which is highly adsorptive and is suitable for chromatographic purposes, especially for thin layer chromatography.

It is a further object of the present invention to provide an aluminium shaped object which, after working up and mechanical stressing, is still outstandingly resistant to corrosion and has good electrical insulation properties.

SUMMARY OF THE INVENTION The present invention provides an aluminium shaped object with an oxidised surface, which is characterised by an anodically produced, elastic aluminium oxide surface layer with a thickness of between 1 and 20 um, preferably of 5 to 6 pm, and with a spongy structure of granule-like structural elements with an average diameter of between 1 and 10 um, preferably of 3 to 8 pm. The aluminium shaped object according to the present invention can have any desired shape but is preferably in the form ofa sheet, plate, foil, wire or rod.

DETAILED DESCRIPTION OF THE INVENTION The production of the aluminium shaped object according to the present invention by anodic oxidation in an electrolyte is carried out by contacting the surface of the aluminium shaped object in a weakly acidic to alkaline bath by sparkovers at high voltages and very high current densities. By means of this contacting or rastering off, each sparkover results in the formation of an insulated, oxidised burnt spot on the aluminium surface so that the next spark flashes over to a still unoxidised spot.

The voltages used are preferably between 100 and 200 V., the lower limit of the striking voltage being regarded as being about 80 to 90 V. The striking voltage depends upon and is inversely proportional to the temperature. Thus, when using the arrangement described hereinafter in more detail in Example 1, there was obtained, with an electrode gap of about 100 mm. and an electrode surface of about cm the following temperature-striking voltage dependence:

TABLE I bath temperature striking voltage T. V.

0 180 200 I0 I40 I 20 I40 In contradis'tinction to the known eloxal process. in the case of the process according to the present invention, no noteworthy current flows below the striking voltage.

The current density used is preferably within the range of 5 to 40 A/dm.

The nature of the electrolytes used is of minor importance. In principle, an electrolyte can be tested, without great difficulty, for its suitability in preliminary experiments so long as it is borne in mind that it must lead to the spark oxidation of the aluminium surface. It is preferred to use neutral to weakly alkaline electrolytes which generally give better surfaces than acidic electrolytes, which can, however, also be used. Weakly acidic electrolytes are, according to the present invention, those with a pH value above 4. Of the neutral to weakly alkaline electrolytes, i.e., electrolytes in the pH range of about 6 to 10, those based on sodium ions have proved to be better than, for example, those based on potassium ions. Especially good results have been obtained with weakly basic sodium salts, for example, sodium carbonate or sodium acetate, in admixture with sodium fluoride.

As already mentioned, the surfaces of the aluminium shaped objects according to the present invention have a spongy structure of granular structural elements, the average diameter of which can be between 1 and 10 um, preferably between 3 and 8 pm. When maintaining the preferred working conditions in the production process, there is obtained an average layer thickness of about 5 um, the dimension of which is only slightly dependent upon the voltage used.

The accompanying drawing illustrates, in FIGS. 1, 2 and 3, typical raster-microscopic pictures of the surface at enlargements of 100, 300 and 1,000 fold. The layers were obtained at a temperature of between about l and 5C. and a voltage of 150 V with an electrolyte which consisted of an aqeuous 2M sodium carbonate and 1M sodium fluoride solution. The extraordinarily uniform spongy structure can easily be seen.

Because of this new type of microstructure of the oxide surface of the aluminium shaped object according to the present invention, there are provided possibilities of use for which the previously known aluminium oxide surface layers were unsuitable.

Of especial interest is the use of the product of the present invention as carrier material and adsorbent for thin layer chromatography. Whereas in the case of the previously known layers suitable for thin layer chromatography, the lower limit of the layer thickness was limited, by various parameters, to about am, it is now possible, with the substantially thinner layers of the present invention, also substantially to reduce the amounts of material needed for carrying out thin layer chromatography and, at the same time, considerably to increase the rate of separation. The detection limits of this analytical method, which is better called thin film chromatography, are, therefore, considerably lowered. Experiments have shown that amounts of substances of g. and less can be separated and detected.

An especial advantage of the present invention is that, in contradistinction to thin layer chromatography on plates, which is the conventional technique, it is now possible to employ wires, strips and other shapes for the aluminium carrier. The use of carriers in the form of wires provides an interesting and new separation technique. After the chromatographic separation has taken place, the substances of interest can then be recovered without loss merely by cutting out the appropriate sections of the wire and dissolving them in, for example, an acid or lye, or the substances of interest can be dissolved off from the carrier with an appropriate solvent.

The product according to the present invention can also be used in electrophoretic, gas chromatographic and partition chromatographic separations as carrier and/or separation materials. Thus, for example, they can be used for liquid-liquid partition chromatography. When used for gas chromatography, an aluminium shaped object according to the present invention can, for example, be introduced, in wire form, into a synthetic resin tube, for example into a tube of TEFLON, to form a stationary phase which can be used for the separation of extremely small amounts of substances. Further possibilities of use as sample carriers include solution X-ray fluorescent analysis and special sampling techniques, for example by the corundum rodlet method.

Very thin layers for thin layer chromatography are already known. These layers are produced by the vapour deposition of metallic oxides, for example of indium oxide or of bismuth oxide, on substrates of, for example, glass. However, these thin films have a limited absorptive ability, are less homogeneous and are difficult to produce.

In contradistinction thereto, the product according to the present invention, when used in thin film chromatography, not only possesses the above-mentioned advantages but also overcomes the difficulties which, in the case of normal thin layer chromatography, arise in connection with the handling and production of the very sensitive layers. These advantages are, to a large extent, also applicable in the case of the use in other chromatographic and electrophoretic methods.

Quite apart from the above-mentioned analytical uses, the product according to the present invention also has surface properties which make it of extreme interest in other technical fields. Thus, an aluminium shaped object, such as an aluminium sheet, eloxated in known manner, has a hard, brittle, crystalline aluminium oxide layer which, in the case of subsequent working up, forms cracks and fissures so that the desired surface protective action is no longer retained. This defect has excluded the use of the eloxal process for surface protection in many technical fields in which a subsequent working up is necessary, for example, in aircraft technology. In these technical fields, the desired corrosion protective action must be achieved by coating with lacquer.

However, products according to the present invention possess aluminium surfaces which are very elastic and can, without diffieulty, withstand a subsequent working up. Furthermore, they also provide an outstanding adhesive substrate for lacquers and colouring materials and, in this regard, too, are also superior to the previously known eloxal layers.

These advantages were ascertained with conventional corrosion tests, for example, the salt-spray test (10% sodium chloride solution at 35C.), as well as the sulphur dioxide test (Kesternich test according to German lndustrial Standard (DIN) No. 500l8, applied to 10 test discs).

In addition. the surfaces of the products according to the present invention are also outstanding insulators. This property enables them to he used for condensers and other electrical constructional elements in which an insulating layer over a conductive substrate material based on aluminium is desired. An especial advantage for this field of use is also the formability of the products according to the present invention.

The following Examples are given for the purpose of illustrating the present invention:

EXAMPLE 1 Production a. There was used an electrolysis vessel equipped with a stirrer, the temperature of which could be regulated by means ofa thermostat. As cathode, there was used an aluminium plate and as anode an aluminium shaped piece, with the shape shown in the following diagram:

The anode was supplied with current via the narrow part thereof. The broad part was completely immersed in the electrolytes used. The cathode to anode surface ratio was 4:1 and the distance between the cathode and the anode was 10 cm.

The surface of the anode was cleaned by immersion for 20 seconds in a 15% aqueous sodium hydroxide solution at C., followed by rinsing with distilled water.

The electrolyte solution used was 2 molar with regard to sodium carbonate and 0.25 molar with regard to sodium fluoride. When applying a sparking voltage of about V to V, a spark discharge occurred in which a spark front migrated over the surface of the al uminium sheet until the whole surface thereof had been oxidised.

The whole of the oxidised aluminium sheet had a very uniform surface layer which had outstanding properties when used as a carrier for thin film chromatographic separation processes.

b. The process according to Example 1(a) was repeated but with the difference that the electrolyte used was 1 molar with regard to sodium fluoride. Here, too. there was obtained a surface layer with outstanding properties.

c. Under otherwise the same conditions as in Example 1(a), instead of a rectangular aluminium anode. there was used a length of aluminium wire (length 10 cm., diameter 1 mm.). After 3 minutes, the wire was uniformly coated with an oxide layer. There was no difficulty in transporting a wire continuously over a spool system, at a constant velocity, through the electrolyte so that the residence time was just sufficient for the complete oxidation of the section of wire present in the bath.

The layers produced according to Example l(a) and (b) had an average thickness of 5 am. Pictures taken with an electron raster microscope enabled a spongy structure of the layer to be recognised with an average grain size of about 5 am.

EXAMPLE 2 Illustration ofa chromatographic separation ofa dyestuff mixture with a shaped body according to the presem invention.

On to a plate produced according to Example 1(a), there was applied a solution of a dyestuff test mixture The detection limit for the above-mentioned organic coloured materials was found to be 0.1 rig. for all of ,them, except titan yellow, the detection limit of which was about 1 ng.

EXAMPLE 3 Use ofa shaped body according to the present invention for the chromatographic separation of metal dithi zonates in the ng. range.

Solutions of the individual dithizonates or of mixtures thereof were dissolved in chloroform and applied in drop form in ng. amounts with capillary pipettes on to a starting line on the plate. After evaporation of the sol vent, the plates were developed with various eluants in small, closed glass vessels. In the following Table Ill. there are summarised the R, values of cadmium, lead, copper, nickel, cobalt and zinc dithizonates for severa different eluants:

in acetone comprising dithizon, a-naphthol-benzoin,

thymol blue and titan yellow, using an ultrami-v cropipette, application being in the form of spots on a starting line on one end of the plate. The plate was developed upwardly in the usual manner in a small ground glass vessel, the lid of which was firmly closed. As eluant I, there was used actone:isopropanol:- chloroform in a volume ratio of4:3:2. 15 to seconds after application of the eluant, the substances were already separated. For a running distance ofabout 2 cm., a running time of 1 to 2 minutes was necessary.

The above process was repeated with the use of elu ant ll, consisting of acetone and ammonia in a ratio of 95:5. With this eluant, too, a complete separation also took place after only a few seconds.

The R, values determined for both eluants are shown in the following Table II:

TABLE u l R, value ll R, value substance 0.87 0.90 a-naphthol-benzoin 0.80 0.82 dithizon metal complex 0.71 0.73 dithizon O.l4 0.27 thymol bluc 0.05 0.10 titan yellow 1 eluant l ll eluant ll Separations of cadmium, cobalt and nickel and of cadmium, cobalt, lead and copper took place satisfac- 45 torily. It was still possible to detect the following with the naked eye:

It was possible to reach somewhat lower limits of detection when the chromatography was carried out on thin wires (diameter 1 mm.) instead of on plates.

Examples 2 and 3 show that, with the spaed objects according to the present invention, it is possible to carry out extremely rapid chromatographic separations 0 of extremely small amounts of substances. The method can also be applied to the separation of small amounts of substances such as are usually carried out with aluminium oxide adsorbents.

We claim:

1. Process for the production of an aluminum shaped object having an oxidized surface by anodic oxidation in an electrolyte which process comprises contacting the surface of an aluminum shaped object in a weakly acidic to alkaline electrolyte bath containing sodium carbonate and sodium fluoride with sparkovers at a spark voltage of more than 80 V. and very high current density.

2. Process as claimed in claim 1, wherein there is used a spark voltage of more than 100 V.

3. Process as claimed in claim 2, wherein there is used a spark voltage of 100 to 200 V.

4. Process according to claim 1, wherein there is used a current density of between and 40 A/dm*.

5. Process for the production of an aluminium shaped object with an oxidised surface by anodic oxidation in an electrolyte, wherein the surface of an aluminium shaped object is contacted in a weakly acidic to weakly alkaline electrolyte bath containing sodium fluoride and a weakly basic sodium salt by sparkovers at a spark voltage of 100 to 200 V and a current density of 5 to 40 A/dm.

6. An aluminum shaped object having an elastic aluminum oxide surface layer with a thickness of between 1 and 20 am. with a spongy structure of granular structural elements of an average diameter of between I and 10 ,um, formed by contacting the surface of the aluminum shaped object in a weakly acidic to alkaline electrolyte bath containing sodium carbonate and sodium fluoride with sparkovers at a spark voltage of more than V. and very high current density.

7. An aluminum shaped object as claimed in claim 6. wherein the aluminum oxide surface layer has a thickness of 2 to l0 am.

8. An aluminum shaped object as claimed in claim 6, wherein the granular structural elements have an average diameter of between 3 and 8 am.

9. An aluminum shaped object as claimed in claim 6.

in the form of a wire sheet, plate. rod or foil. 

1. PROCESS FOR THE PRODUCTION OF AN ALUMINUM SHAPED OBJECT HAVING AN OXIDIZED SURFACE BY ANODIC OXIDATION IN AN ELECTROLYTE WHICH PROCESS COMPRISES CONTACTING THE SURFACE OF AN ALUMINUM SHAPED OBJECT IN A WEAKLY ACIDIC TO ALKALINE ELECTROLYTE BATH CONTAINING SODIUM CARBONATE AND SODIUM FLUORIDE WITH SPARKOVERS AT A SPARK VOLTAGE OF MORE THAN 80 V. AND VERY HIGH CURRENT DENSITY.
 2. Process as claimed in claim 1, wherein there is used a spark voltage of more than 100 V.
 3. Process as claimed in claim 2, wherein there is used a spark voltage of 100 to 200 V.
 4. Process according to claim 1, wherein there is used a current density of between 5 and 40 A/dm2.
 5. Process for the production of an aluminium shaped object with an oxidised surface by anodic oxidation in an electrolyte, wherein the surface of an aluminium shaped object is contacted in a weakly acidic to weakly alkaline electrolyte bath containing sodium fluoride and a weakly basic sodium salt by sparkovers at a spark voltage of 100 to 200 V and a current density of 5 to 40 A/dm2.
 6. An aluminum shaped object having an elastic aluminum oxide surface layer with a thickness of between 1 AND 20 Mu m. with a spongy structure of granular structural elements of an average diameter of between 1 and 10 Mu m, formed by contacting the surface of the aluminum shaped object in a weakly acidic to alkaline electrolyte bath containing sodium carbonate and sodium fluoride with sparkovers at a spark voltage of more than 80 V. and very high current density.
 7. An aluminum shaped object as claimed in claim 6, wherein the aluminum oxide surface layer has a thickness of 2 to 10 Mu m.
 8. An aluminum shaped object as claimed in claim 6, wherein the granular structural elements have an average diameter of between 3 and 8 Mu m.
 9. An aluminum shaped object as claimed in claim 6, in the form of a wire, sheet, plate, rod or foil. 